The present invention relates generally to x-ray producing equipment. More particularly, the invention relates to an improved anode target structure present on an x-ray tube of the sort that is commonly used in such x-ray producing equipment. In addition, the present invention relates to a method of manufacturing an improved anode target structure for use in an x-ray tube.
X-ray producing devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. Such equipment is commonly used in areas such as diagnostic and therapeutic radiology; semiconductor manufacture and fabrication; and materials testing.
The basic operation for producing x-rays in the equipment used in these different industries and applications is very similar. X-rays, or x-radiation, are produced when electrons are produced and released, accelerated, and then stopped abruptly. Typically, this entire process takes place in a vacuum formed within an x-ray generating tube. An x-ray tube ordinarily includes three primary elements: a cathode, which is the source of electrons; an anode, which is axially spaced apart from the cathode and oriented so as to receive electrons emitted by the cathode; and some mechanism for applying a high voltage for driving the electrons from the cathode to the anode.
The three elements are usually positioned within an evacuated glass tube, and connected within an electrical circuit. The electrical circuit is connected so that the voltage generation element can apply a very high voltage (ranging from about ten thousand to in excess of hundreds of thousands of volts) between the anode (positive) and the cathode (negative). The high voltage differential causes a thin stream, or beam, of electrons to be emitted at a very high velocity from the cathode towards an x-ray xe2x80x9ctargetxe2x80x9d positioned on the anode. The x-ray target has a target surface (sometimes referred to as the focal track) that is comprised of a refractory metal. When the electrons strike the target, the kinetic energy of the striking electron beam is converted to electromagnetic waves of very high frequency, i.e., x-rays. The resulting x-rays emanate from the anode target, and are then collimated for penetration into an object, such as an area of a patient""s body. As is well known, the x-rays that pass through the object can be detected and analyzed so as to be used in any one of a number of applications, such as x-ray medical diagnostic examination or material analysis procedures.
In general, a very small part of the electrical energy used for accelerating the electrons is converted into x-rays. The remainder of the energy is dissipated as a large amount of heat in the target region and the rest of the anode. This heat can damage the anode structure over time, and can negatively affect the operating life of the x-ray tube and/or the performance and operating efficiency of the tube. To alleviate this problem the x-ray target, or focal track, is typically positioned on an annular portion of a rotatable anode disk. The anode disk (also referred to as the rotary target or the rotary anode) is mounted on a supporting shaft that is rotated by a motor. The motor is used to rotate the disk at high speeds (often in the range of 10,000 RPM), thereby causing the focal track to rotate into and out of the path of the electron beam. In this way, the electron beam is in contact with specific points along the focal track for only short periods of time, thereby allowing the remaining portion of the track to cool during the time that it takes the portion to rotate back into the path of the electron beam.
While the rotation of the track helps reduce the amount and duration of heat dissipated in the anode target, the focal track is still exposed to very high temperaturesxe2x80x94often temperatures of 2500xc2x0 C. or higher are encountered at the focal spot of the electron beam. Thus the rotary anode must still be constructed of a material that is both resistant to heat, and that can effectively block an impinging high velocity electron beam. Moreover, since the disk is rotated at high rotational speeds, it must be capable of withstanding high mechanical stresses. One commonly used material for an anode disk is a refractory metal, such as a molybdenum alloy, an example of which is known as TZM (titanium-zirconium-molybdenum). Refractory metals are, however, expensive, and require complex manufacturing and processing procedures to be used for fabrication of an anode disk. Also, such metal alloys are quite dense and thus can be very heavy, which can be especially problematic when a larger anode disk is used. For instance, the higher weight requires a larger motor and stronger rotor assembly to rotate the anode disk, resulting in higher costs, and greater wear and tear on the components. Moreover, the increased weight of a metal anode disk makes it more difficult to rotate at high speeds, especially in x-ray devices that require the anode disk to be accelerated quickly to high operational speeds in short periods of time.
One approach to address the problems encountered when a refractory metal is used, has been to use a graphite material. Graphite offers several advantages over metal. It has a significantly higher heat storage capacity than metal, and thus can operate at higher temperatures for longer periods of time. Graphite also has a much lower density (lighter weight) than metal, so it can be more easily rotated at higher speeds, allows for the use of bigger targets, and puts less mechanical stress on the anode assembly (such as the rotor, bearings and motor).
Graphite, however, has a low mechanical strength and can be brittle, especially pressed and sintered graphite. As such, mechanical loadingxe2x80x94for example, tangential loading during starting and stopping of rotationxe2x80x94can cause fracturing of the graphite disk, especially with the high rotational speeds encountered by the rotating anode. Also, a focal track constructed of a material that is capable of blocking an impinging high velocity electron beam must be applied directly to the graphite substrate. Typically, this results in an anode where the rate of heat transfer from the focal track to the substrate is slower than when a focal track is attached to a metal substrate, such as TZM. Under certain operating conditions, this can cause an overheating of the focal track and resultant damage to the graphite target disk, such as bonded layer failure.
It has also been proposed that a carbon-carbon composite material be used in place of graphite. Such a material exhibits the same heat storage capacity and low weight characteristics of graphite, but is much stronger than graphite, and is better able to withstand the mechanical stresses imposed. As with graphite, a suitable metal material must be bonded to the carbon-carbon disk to function as the anode focal track. The material must be of sufficient thickness so as to effectively block an impinging high velocity electron beam and generate usable x-ray output, and must also be capable of withstanding the high temperatures that are dissipated on the track during operation. At the same time, the focal track material must remain bonded to the underlying carbon-carbon composite disk. This gives rise to the primary problem with the carbon-carbon material, in that its thermal expansion rate differs significantly from the metal materials that are commonly used for the focal track on the disk. Maintaining a bond is thus difficult to achieve. When exposed to high temperatures, the different thermal expansion rates result in a macroscopic buildup of stresses across the bonding surface between the focal track target material and the carbon-carbon composite material. These stresses often result in a debonding, peel-off, or cracking of the target layer, which can render the x-ray tube inoperable, shorten its operating life, or reduce its operating efficiency.
As such, there is a need in the art to provide a rotating anode disk that is constructed of a material that has a low density and is a light weight. The disk should also have a high heat storage capacity and be capable of being used in extremely high heat conditions. In addition, the disk should be capable of withstanding the high mechanical stresses encountered at high rotational speeds. Moreover, it would be desirable to have a disk structure that can be used in connection with a refractory metal target surface that is capable of stopping an impinging high velocity electron beam so as to produce x-rays in an efficient manner. Finally, the bond between the refractory metal target surface and the underlying disk substrate material should be capable of withstanding the stresses that result from the different rates of thermal expansions of the two materials when they are together subjected to high temperature conditions.
It is, therefore, a primary object of the present invention to provide an improved rotating anode for use in connection with an x-ray tube and x-ray generating system.
It is another object of the present invention to provide a rotating anode that is constructed of a substrate material that has a low density and that is light in weight.
It is still another object of the present invention to provide a rotating anode that is constructed of a substrate material that is durable and resistant to cracking or other catastrophic failure, even when subjected to extremely high rotational speeds.
It is yet another object of the present invention to provide a rotating anode that is capable of being subjected to the high thermal stresses that are present in an operating x-ray tube.
It is an even further object of the present invention to provide a rotating anode that utilizes a focal track that can be thermally and mechanically bonded to a carbon-carbon composite substrate material and that remains attached even when exposed to high operational temperatures.
Yet another object of the present invention is to provide a method for manufacturing a rotating anode that achieves the foregoing objectives.
In accordance with the invention as embodied and broadly described herein, the foregoing and other objectives are achieved by the present invention, which is directed to an improved rotary anode for use within an X-ray tube of the sort that is commonly used in x-ray producing systems. Further, the invention is directed to a novel method for manufacturing the improved rotary anode. In general, the present invention is directed to an improved rotary anode that is constructed of a carbon composite material, which in a presently preferred embodiment is a carbon-carbon composite material. This composite is particularly suitable for use as a rotating anode material. The material has a low density, and thus is very light in weight. This permits the construction of a rotating anode that is also light in weight, even when built in larger dimensions. As such, the anode can be more easily rotated and accelerated to the high operational speeds that are common in many x-ray systems and applications. Also, the lighter weight characteristics mean that the operational speeds can be obtained without requiring a larger motor, and without requiring a stronger rotor and bearing assembly. This reduces the overall cost of the x-ray tube system. Moreover, the material is extremely strong and durable, and remains so in the presence of extremely high temperatures. Further, the material dissipates heat efficiently, and thus allows a rotating anode to remain sufficiently cool during extended periods of operation.
In addition, the improved anode includes a focal track which is comprised of conventional metallic materials that are capable of efficiently generating x-rays when contacted with a high speed electron stream. In the anode of the present invention, such focal track materials are capable of being thermally and mechanically coupled to the carbon composite disk substrate, even though they exhibit rates of thermal expansion that are different from that of the underlying carbon substrate. This capability is provided by way of an interface means, that is disposed between the surface of the carbon anode disk and the target track material, that functions so as to diffuse interfacial stresses that occur between the track layer and the carbon composite substrate during thermal expansion of the two materials. Because these stresses are diffused, the track layer remains bonded to the carbon substrate, even when exposed to the extremely high temperatures present during the operation of an x-ray tube.
In one presently preferred embodiment, the interface means is comprised of a bond interface layer that is formed on the top surface of the carbon composite substrate material. More particularly, this interface layer is produced by microscopically roughening the surface of the substrate in a manner such that it structurally exhibits, for instance, as series of peaks and valleys similar to a xe2x80x9csaw-toothxe2x80x9d-like configuration. This provides a high surface contact area per unit length, and diffuses any shear stresses that occur between the track layer and the composite substrate during thermal expansion and/or contraction.
In a preferred embodiment, the bond interface is formed on the surface of the carbon composite by removing carbon atoms from the surface. This removal of carbon atoms produces the above-mentioned xe2x80x9csaw-toothxe2x80x9d-like arrangement. While removal of carbon atoms can be accomplished using various techniques, in a preferred embodiment it is accomplished by thermally etching (oxidizing) the surface of the carbon-carbon composite substrate. The carbon composite is comprised of both carbon fibers and carbon matrix, and the oxidation process removes carbon atoms from the fibers and the matrix at different rates, thereby producing the roughened surface.
In addition to providing an improved bond interface, the saw tooth arrangement also provides additional benefits. In particular, the composite material possesses improved thermal emissivity characteristics. This allows the rotating anode to cool down more efficiently, thereby permitting it to be operated at higher temperatures for longer periods of time.
Additional objects, features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.